1. Field of the Invention
The present invention relates to a removable battery pack for providing power to a system, and more particularly to a removable battery pack that uses the same transistor to limit the voltage to its batteries and to block current to the batteries.
2. Description of the Related Art
Computers are often needed in locations where AC power is not available. Rechargeable batteries are typically used as an alternative source of power, such as nickel-based batteries and lithium ion batteries, which are capable of providing power to a portable or pen-based computer system for several hours. For providing power to a computer system, these batteries are typically arranged in a battery pack containing four series banks of two parallel cells in each battery. Although the battery pack may be charged in an external charger, the battery pack of a computer system is typically charged by the power supply of the host computer system. Since rechargeable batteries have a limited cycle life, it is desirable to maximize the life of and to obtain the maximum power from each battery during every discharge cycle. Thus, in order to achieve these goals, it is necessary to fully and efficiently charge the battery pack in a system designed to accomplish these objectives.
The primary design challenge confronting the earlier nickel-based battery chargers arose from the difficultly in determining the charge level of a nickel-based battery since the terminal voltage of the nickel-based battery is substantially the same regardless of the charge level. This characteristic also holds true for lithium ion batteries. This problem was addressed by placing a microcontroller circuit and a memory inside a battery pack as disclosed in U.S. Pat. No. 5,315,228 entitled "Battery Charge Monitor and Fuel Gauge," which is hereby incorporated by reference. In that disclosure, the battery pack recalculates the remaining capacity of the battery pack over time; provides a fuel gauge to continually measure the remaining charge level at any given time; and measures the discharge of the battery pack during periods of non-use, often referred to as the self-discharge of the battery pack.
A further modification of this arrangement is disclosed in U.S. patent application Ser. No. 033,821 entitled "Battery Pack Including Static Memory and a Timer For Charge Management," filed Mar. 3, 1993 and allowed on Jul. 28, 1995, which is hereby incorporated by reference. A battery pack is disclosed in which the microcontroller is moved to the host computer system, thereby reducing the size and cost of the battery pack and increasing the battery pack shelf-life. The battery pack includes a small read only memory (ROM), a random access memory (RAM), and a real time clock. The operating parameters of the battery pack are contained in the ROM memory which includes a family code and battery type, the maximum charge temperature, low voltage set points, initial rated total capacity, and self-discharge rates for several time periods. The RAM includes such information as the remaining charge on the battery and the total capacity of the battery. Furthermore, the real time clock timestamps the RAM, thereby providing a measure of the elapsed time of removal of the battery pack from the host computer system. Thus, the host computer system has the appropriate information to provide optimal charge to the battery pack and to monitor the charge left in the battery pack.
One fundamental requirement of lithium ion batteries is that they be charged or discharged only when they are inserted in a known computer system or special charger designed to accommodate lithium ion batteries. This requirement is predominantly due to potential for a lithium ion battery to possibly explode if improperly charged or discharged.
Thus, it is desirable to have a lithium ion battery pack which disables itself, by blocking charge entering or leaving the batteries, if the battery pack is not inserted into a known system. If the battery pack is inserted into an unknown system, (such as a short circuit), an unknown battery charger, or an unknown computer system, the battery pack will minimize the harm to the computer system, charger, or batteries of the battery pack. It is further desirable to have a battery pack which permanently disables itself should the terminal voltage of any battery cell of the battery pack drop below a deeply discharged voltage threshold which indicates a deeply discharged battery cell, thereby not allowing any recharging or discharging of the battery pack, as this would be dangerous as known to those skilled in the art.
Further, because battery packs are used in portable computers, any size reduction yields benefits. One technique for reducing battery pack size is to reduce the battery pack's component count. Therefore, it would be desirable to eliminate components in a lithium-ion battery pack.